The present invention relates to a suction apparatus for introducing outer air into a cylinder chamber of an engine through an intake path and, more particularly, to a suction apparatus for an engine, which comprises a resonator for eliminating a resonance wave induced in an intake path by an intake pulsation, and can eliminate intake noise and can improve intake air filling efficiency.
In an engine, a variation in pressure, i.e., an intake pulsation periodically occurs near an intake port by an intake negative pressure generated when an intake valve is opened. For example, a four-cylinder engine which comprises a surge tank which collects independent intake paths of the cylinders, and also comprises a common intake path for introducing intake air into the surge tank is known. In the four-cylinder engine, the surge tank is an intake pulsation generation source. A compression wave of intake air is generated by the intake pulsation. The compression wave is transmitted at a sonic speed toward the upstream side in the common intake path, is reflected at an upstream opening portion (to be simply referred to as an opening portion hereinafter) of the common intake path, and is then transmitted to the surge tank toward the downstream side in the common intake path. In the following description, the compression wave which propagates toward the upstream side in the intake path will be referred to as an incident wave hereinafter, and the compression wave which propagates toward the downstream side will be referred to as a reflection wave hereinafter.
More specifically, the opening portion is defined as a transition region from a high-impedance region in the common intake path to a zero-impedance region in the air. As a result, the opening portion serves as a reflection surface of the incident wave. At the reflection surface, the impedance changes from high to low along the incident direction of the compression wave. Therefore, the incident and reflection waves have opposite compression states (i.e., have a 180.degree. phase difference). The reflection wave has the same wavelength and amplitude as those of the incident wave, and has an opposite propagation direction. As a result, as is well known as synthesis of two waves, the incident and reflection waves interfere with each other, thus forming a standing wave having the same wavelength as that of the incident/reflection wave in the common intake path.
When the relationship given by the following equation is established between a path length L of the common intake path and a wavelength .lambda. of the standing wave: EQU L=(2n+1).lambda./4
intake air in the common intake path resonates to have the surge tank (intake pulsation generation source) as a loop and the opening portion as a node with respect to a sound pressure, and a large, i.e., (0.5+n)th-order standing wave is formed. This standing wave will be referred to as a resonant wave hereinafter. Upon generation of the resonant wave, large noise is undesirably generated from the opening portion.
When the relationship given by the following equation is established between the path length L and the wavelength .lambda.: EQU L=2n.lambda./4
intake air in the common intake path resonates to have the surge tank and the opening portion as nodes with respect to a sound pressure, and a large, i.e., nth-order standing wave is formed. Upon generation of this standing wave, large noise is generated from the opening portion.
In order to eliminate the intake noise, as disclosed in, e.g., Japanese Patent Laid-Open (Kokai) Nos. 54-9316 and 61-190158, suction apparatuses are proposed. In these apparatuses, a side-branch type or Helmholtz type resonant silencer is arranged at a position near the opening portion and where the loop of a sound pressure vibration of the resonant wave is formed (normally, at the upstream side of an air cleaner), thereby controlling the sound pressure vibration of the resonant wave induced in the common intake path and eliminating the intake noise.
In the (0.5+n)th-order resonant wave generated in the common intake path, the loop of the sound pressure is formed at the downstream end portion (the opening portion to the surge tank). As a result, the intake pressure in the surge tank vibrates with a large amplitude upon vibration of the sound pressure (intake pressure). The vibration of the intake pressure becomes a minimum pressure when a 3/4 period elapses from when the compression wave generated at the intake pulsation generation source has a minimum pressure (at substantially the same time with an opening timing of the intake valve).
More specifically, in the four-cylinder, four-cycle engine, the period of the compression wave corresponds to a crank angle of 180.degree.. For this reason, when the crank angle after the intake valve is opened reaches 135.degree. (=180.times.3/4) i.e., at the end of an intake stroke, the intake pressure in the surge tank becomes minimum. In general, since the intake path length between each cylinder and the surge tank is relatively short, a change in pressure in the surge tank is transmitted to each cylinder without a delay time. For this reason, in an engine rotational speed region corresponding to the wavelength .lambda. at which the resonant wave is generated in the common intake path, intake air filling efficiency is impaired, and an output torque is decreased.
The nth-order resonant wave does not induce the intake pressure vibration in the surge, tank since the downstream end portion (surge tank) of the common intake path becomes the node of the sound pressure.
The conventional resonant silencer which is arranged to eliminate intake noise eliminates the resonant wave in the common intake path. Therefore, a negative pressure induced in the surge tank by the (0.5+n)th-order resonant wave can be theoretically eliminated upon operation of the resonant silencer. However, as described above, the resonant silencer is mainly arranged near the upstream opening portion of the common intake path for the purpose of eliminating intake noise. As a result, although the intake noise can be eliminated, since the silencer is spaced apart from the surge tank, the resonant wave elimination effect does not reach the surge tank.